/*
 * acm1337.c
 *
 *  Created on: 20 Mar 2010
 *      Author: xuxu
 */

#include<stdio.h>
#include<math.h>
#define mmax(a,b) a>b?a:b
#define mmin(a,b) a<=b?a:b

int finder(int large,int smail)
{
	int i;
	for(i=2;i<=smail;i++)
	{
		if(large%i==0&&smail%i==0)
			return 1;
	}
	return 0;
}

int main()
{
	int i,j;
	int n;
	int num[51];
	int con1,con2;
	double pi;
	while(scanf("%d",&n)&&n)
	{
		for(i=0;i<n;i++)
			scanf("%d",&num[i]);
		con1=0;
		con2=0;
		for(i=0;i<n;i++)
		{
			for(j=i+1;j<n;j++)
			{
				if(finder(mmax(num[i],num[j]),mmin(num[i],num[j])))
					con1++;
			}
			con2+=i;
		}

		con1=con2-con1;
		if(0==con1)
			printf("No estimate for this data set.\n");
		else
		{

			pi=sqrt(6*(double)con2/(double)con1);
			printf("%.6lf\n",pi);
		}
	}
	return 0;
}
